Ozone is a toxic gas produced in photochemical air pollution as a result of a complex sequence of reactions involving oxides of nitrogen, hydrocarbons and sunlight. The Clean Air Act in the U.S. and similar laws in other countries set limits on ozone concentrations in ambient air. Enforcement of compliance with the U.S. National Ambient Air Quality Standard requires continuous monitoring of ozone concentrations at hundreds of locations, especially during summer months. Compliance monitoring currently is done almost exclusively by the method of UV absorbance of the Hg emission line at 254 nm. Low pressure mercury lamps provide an intense, stable and inexpensive source of radiation at a wavelength very near the maximum in the ozone absorption spectrum.
It is well known that ozone monitors based on UV absorbance suffer from interferences from other species that absorb at 254 nm. Volatile organic compounds (VOCs) that interfere are generally aromatic compounds. Some VOCs have a larger response at 254 nm than ozone itself. For example, Kleindienst et al. (1993) reported that the response of 2-methyl-4-nitrophenol is about 40% higher than ozone.
Mercury provides a particularly strong interference because the electronic energy levels of Hg atoms are resonant with the Hg emission line of the low pressure Hg lamp used in ozone monitors. The relative response to Hg as compared to ozone depends on the temperature and pressure of the lamp and on the efficiency with which the instrument's internal ozone scrubber removes mercury, but is usually in the range 100-1000. The U.S. Environmental Protection Agency (EPA, 1999) reported that at a baseline ozone concentration of approximately 75 parts per billion (ppb), the action of 0.04 ppb Hg (300 ng/m3 at room temperature) caused an increase in measured ozone concentration of 12.8% at low humidity (RH=20-30%) and 6.4% at high humidity (RH=70-80%) using a UV photometric ozone monitor. For dry air, Li et al. (2006) found that 1 ppb of mercury gave a response equal to approximately 875 ppb of ozone in the same model of Thermo Electron Corporation photometric ozone monitor used in the EPA study. This mercury interference can be quite large inside buildings where mercury vapor may be present as a result of past mercury spills (broken thermometers, fluorescent light fixtures, electrical switches, etc.), near mining operations and near various industrial facilities.
Another way in which UV-absorbing compounds and mercury interfere in the measurement of ozone using ozone photometers is by adsorption and desorption from the instrument's internal ozone scrubber. These scrubbers are typically composed of manganese dioxide, charcoal, hopcalite, metal oxide screens or heated silver wool. UV-absorbing species will adsorb to and accumulate on the surfaces of the scrubber material. If the temperature of the scrubber increases, or if the humidity changes, these species may be released from the scrubber and enter the gas stream. While removal of a UV-absorbing compound from the sample stream by the scrubber will cause a positive interference, subsequent release of UV-absorbing species from the scrubber will cause a negative interference. Because UV-absorbing compounds and mercury are present at some level in all outdoor and indoor air, this interference may be responsible for much of the baseline drift that occurs in photometric ozone monitors.
Water vapor is known to be a significant interference in the measurement of ozone by UV absorbance (Meyer et al., 1991; Kleindienst et al., 1993; Kleindienst et al., 1997; Leston and Ollison, 1993; Leston et al., 2005; Hudgens et al., 1994; Maddy, 1998; Maddy, 1998; Wilson and Birks, 2006). The cause of this interference has been attributed to a difference in the transmission of light through the UV absorbance cell during measurements of light intensity of sample air while bypassing or passing through an ozone scrubber (Wilson and Birks, 2006). The ozone scrubber alters the concentration of water vapor within the detection cell by either removing or adding water to the air stream, depending on the recent history of exposure to water vapor. It has been shown that the water vapor interference can be eliminated by use of a DewLine™, which consists of a short length of Nafion® tubing (or multiple tubes in parallel) attached to the inlet of the detection cell (Wilson and Birks, 2006). Nafion® has the property of rapidly and selectively transporting water molecules. Matching the humidity levels of ozone-scrubbed and unscrubbed air by passing through a DewLine™ just prior to entering the absorbance cell effectively eliminates the humidity interference in UV absorbance measurements (Wilson and Birks, 2006). However, Nafion® tubes are restrictive to air flow and become contaminated over time when sampling ambient air and are expensive to replace. For these reasons, it would be desirable to solve the humidity interference problem in UV absorbance measurements of ozone without the use of Nafion® tubes.
The use of a gas-phase scrubber was recently introduced as a means of eliminating interferences from both UV-absorbing species and water vapor (Birks et al., 2013). In this approach, the solid-phase scrubber is replaced with a gas phase scrubber such as nitric oxide. The disadvantage of this approach is that a source gas is required, reducing the portability of the instrument. For applications requiring highly portable instruments or where use of a gas scrubber is prohibitive, a solid phase scrubber that effectively destroys ozone (≥99% destruction) but efficiently passes UV-absorbing compounds and water vapor is highly desirable.